The disclosures herein relate generally to computer systems, and more particularly to riser cards for memory and the like in a computer system.
With current memory technology from RAMBUS Incorporated, there are two separate system memory architectures used in workstation system designs. Both of these architectures utilize RAMBUS memory technology called RAMBUS In-line Memory Modules (hereafter referred to as RIMMs). One architecture is a xe2x80x9clow-memoryxe2x80x9d scheme and the other is a xe2x80x9chigh-memory schemexe2x80x9d. The low-memory scheme is typically of a lower cost than is the high-memory scheme. Due to the specific nature of each scheme, the components used in the two schemes are not readily interchangeable.
The high-memory scheme is broken into several configurations whereby the high-memory scheme is scaleable. However, the low-memory scheme is not readily scaleable. The scaleability of the high-memory scheme allows each of its configurations to be cost optimized for the amount of memory that it supports. The scaleability of the high-memory scheme assists in addressing cost issues associated with the high-memory scheme.
The low-memory scheme supports 32 RAMBUS devices per channel for a total of 512 MegaBytes per channel. The RAMBUS memory devices are provided on a RIMM. The RIMM is then plugged into a RIMM connector. If more system memory is required, a different memory scheme must be used.
For high-memory schemes, one or more Memory Repeater Hubs (hereafter referred to a MRHs) are mounted on a riser card that is plugged into a riser card connector on the motherboard. Each MRH provides two RAMBUS channels for data processing. Based on current 64 MegaByte memory devices, each channel can support RIMMs totaling 512 MegaBytes per channel. A riser card that supports 1 GigaByte of memory requires only one MRH. A riser card that supports 2 GigaBytes of memory requires two MRHs. MRH""s and detailed information on the operation and applications for MRHs are readily available from Intel Corporation. As higher density devices (i.e. 128 MegaByte) become available, the total amount of memory that each channel can support will increase accordingly.
The riser card is plugged into a riser card connector on the motherboard. The riser card connector provides a RAMBUS channel as well as power to the riser card. Presently, a riser card connector that also meets the impedance requirements of RIMMs does not exist.
Impedance mismatch is a key factor in riser connectors not being compatible with RIMMs in high speed applications. RAMBUS memory technology is based on circuitry with a characteristic impedance of 28 ohms with a tolerance of +/xe2x88x9210%. As a result, RIMM connectors are designed to provide a characteristic impedance of 28 ohms so that it meets the requirements of RAMBUS memory technology. A typical riser connector has a characteristic impedance of 72 ohms. Employing a riser connector in a circuit having a characteristic impedance of 28 ohms would result in an impedance discontinuity. Anytime a signal is propagated down a transmission line having an impedance discontinuity, a reflection is generated. Reflections are not desirable as they induce noise into the signal. In the case of a RIMM operating at high speeds, reflections should be avoided at any cost.
Another issue that will need to be resolved for the scaleability of system memory is power consumption of memory devices on a riser card. The power requirements of an 8 RIMM riser card can be as much as 40 watts as each RIMM requires approximately 5 watts. A standard 184 pin RIMM connector can provide a maximum of 13 amps distributed across 26 power pins. As RIMMs are designed to be operated at 2.5 volts DC (VDC), an 8 RIMM riser card would require as much as 16 amps. This would exceed the recommended maximum amperage rating for a RIMM connector and compromise the reliability of the riser-to-connector power connections.
U.S. Pat. No. 5,604,871 to Pecone discloses a personal computer system utilizing a simplified motherboard having connectors on the motherboard and a riser card or cards having the desired interface connectors and logic circuits thereon. The present invention provides for operatively and removably coupling a plurality of I/O expansion cards, host local bus interfaces and future system upgrades for the computer system without burdening the base cost thereof. The computer system may be expanded or upgraded at any time during manufacture or in the field. A riser card is configured for the desired features, plugged into the motherboard connectors, and a desired new peripheral feature is plugged into the riser card to complete the upgrade.
U.S. Pat. No. 5,604,871 to Cusato et al discloses a computer file server with a specially designed planar/riser card assembly is mounted on the computer chassis and includes a card cage structure in which a riser card is secured. Carried on the riser card are I/O card edge connectors into which all of the system I/O cards may be plugged. The riser card is hard connected into the system and has an edge connector portion. The system planar board is carried on a mounting plate which is screwed to an outer side of the cage structure. A connector carried on the planar board receives the edge connector portion of the riser card, thereby electrically coupling the planar board to the riser card. The riser card serves as a wiring plane containing only the signals which the planar board would normally provide to the I/O cards through xe2x80x9con planarxe2x80x9d connectors.
U.S. Pat. No. 5,524,232 to Hajeer discloses a computer memory module adaptor configured for adapting a computer memory module connector to receive a plurality of memory modules. The memory module adaptor includes a board with connecting pins for electrically connecting the adaptor to the computer memory module receiving connector, a plurality of support members mounted on the memory module adaptor for receiving 8-bit memory modules, and a circuit for adapting the 8-bit memory modules for use as a single higher-order parallel bit memory module.
While these references disclose memory module connectors and riser cards for interconnecting items such as memory cards, they do not address the impedance or power issues associated with using RIMM modules on a riser card in high speed applications. The references disclose riser cards employ conventional connector usage that is limited for effectively and economically addressing scaleability issues for both low memory and high memory schemes, addressing power requirements of the riser card that would result in the connector being subjected to current in excess of the maximum recommended current, addressing scaleability issues for RIMM low-memory and high-memory schemes, or power management issues associated with current requirements in excess of the maximum recommended current for a connector.
Accordingly, a need has arisen for an apparatus that is configured to overcome the shortcomings of prior art riser card techniques and constructions. In particular, there is a need for a cost-effective yet reliable apparatus for providing a riser card that addresses the scaleability needs for both RIMM low-memory and high-memory schemes. This apparatus will employ a connector that is compatible with a RIMM and a riser card. The apparatus will be capable of managing the power requirements of the riser card without drawing a current through the connector that would exceed the maximum recommended current for the connector. The connector will also be compatible with RAMBUS circuitry from an impedance standpoint. The characteristic impedance of the connector used with a riser card according to the present invention will substantially match that of a RIMM.
One embodiment, accordingly, provides a riser card that offers a cost-effective and reliable technique for addressing the scaleability needs for both RIMM low-memory and high-memory schemes. The riser card may be plugged into a connector that is compatible with RIMM modules as well as riser cards designed for RIMM connectors. The riser card is configured such that the amount of current drawn through the connector for operating the riser card is maintained at or below the maximum rated amperage of the connector. To this end, a computer system having a power supply system and a motherboard includes a memory connector mounted on the motherboard. The memory connector is configured to receive a memory module. A riser card having an edge portion is configured to be received by the memory module connector. The riser card is removably inserted into the memory connector. A riser card interface is coupled from the riser card connector to the power supply system for enabling an interface-dependent voltage to be established by the power supply system. The interface-dependent voltage is supplied to the riser card through the memory connector and is maintained at a level whereby the memory connector is subjected to an electrical current less than a maximum prescribed current.
A principal advantage of this embodiment is that the riser card allows the low-memory scheme and high-memory scheme to be interchangeable in any given computer system. In the low-memory scheme, RIMM""s can be plugged directly into a RIMM compatible connector. In the high-memory scheme, a riser card carrying an MRH is plugged into the RIMM connector such that additional memory can be provided. As the riser card carries a voltage regulator, the current drawn through the connector for operating the memory may be managed such that it does not exceed that maximum rated current of the connector.